The following publications are representative of the most relevant prior art known to applicant as of the time of the filing of this application:
______________________________________ U.S. Pat. 2,899,286 August 11, 1959 Miller 3,902,856 September 2, 1975 Burroughs et al. 4,229,418 October 21, 1980 Wijffels et al. 4,590,045 May 20, 1986 van der Wal et al. ______________________________________